To improve the separation from earth, of a rotary work machine, by preventing the lowering of PTO rotational frequency, when the engine speed is changed and lowered.

A TOP clutch 27 and a clutch 20 for accelerating are installed on a PTO transmission system of a tractor, separately from a PTO change gear part 15. An accelerating part 23 for transmitting the rotation of a PTO input axis 13 to the PTO gear change part 15 through the engagement of the gears 21, 22, when the clutch 20 for accelerating is 'IN', is formed. The TPO rotational frequency is accelerated in the case through the accelerating part 23, in comparison with the case through the PTO clutch 27, to be transmitted to the PTO gear change part 15. By accelerating the PTO rotational frequency by the accelerating part 23, when the engine speed is changed and is slightly lowered, the separation from earth, of the rotary work machine can be improved without the stop of engine, and the involving of earth can be prevented.